There is a need to conserve water, which is in short supply in many areas of the world. Most water treatment processes remove undesired impurities. For example, anions and cations may be removed from water by chemical exchange in water softening, and by what is effectively a filtration technique in reverse osmosis.
In reverse osmosis processes, the waste stream of water often represents a large percentage of the raw feed water stream. Most small, residential reverse osmosis units produce about 5 liters of purified water for every 25 liters of feed water. The remaining 20 liters of water contain concentrated impurities which have been left behind in the main stream, as the purified 5 liters of water have been drawn off.
In the prior art, attempts have been made to avoid the waste of this byproduct water from reverse osmosis. For example, the WaterPure.SM. system marketed by BioLab Inc. of Decatur, Ga. is a system which recycles the waste water from a reverse osmosis apparatus for household use in secondary faucets, toilets, and appliances, from which drinking of the water is not expected. Thus, all of the water can be used, without waste.
In the WaterPure system, the entire water supply may pass through a reverse osmosis (R. O.) module and/or a water softener. However, to accomplish this, substantial plumbing parts may be required for installation of the system. Particularly, it may be difficult or impossible to install such a system under a kitchen sink or the like, since one must reroute the main water line, which is often a 3/4 inch pipe, under the kitchen sink to the R. O. module and then back out again.
On the other hand, the WaterPure apparatus may be installed in connection with a branch line from the main water line, using 3/8 inch water line or the like to communicate with the reverse osmosis module, and then to convey the product water to a drinking water storage tank while also to conveying waste water to a water faucet or the like from which drinking is not intended.
However, while such a system with smaller water lines can be more easily installed under a sink or the like without major plumbing work, and the supply of drinking water produced by such a system may be adequate for a single family, the use of waste water from the module at a single tap may be inadequate to provide enough flow to clear the R. O. module of precipitating waste products, resulting in a phenomenon called "scaling", which can render the module inefficient or inoperative.
Accordingly, there is a need for a water treatment system, for example a reverse osmosis system, which can be easily installed without major plumbing modifications as would be involved in the rerouting of 3/4 inch main lines, while at the same time providing sufficient flow of the waste stream by routing it to several outlets, thereby avoiding scaling by increasing the overall flow through the R. O. module.
In Menon et al. U.S. Pat. No. 5,006,234, a reverse osmosis water purification system is provided in which water is tapped off of a main water supply line and passed through a reverse osmosis unit, with the waste water being returned again to the main water supply line. A pressure differential is provided between the point of tapping off of the water and the point of return of the water, to drive water flow through the reverse osmosis unit.
However, the system of Menon et al. is highly subject to the "scaling" phenomenon previously mentioned, which renders the reverse osmosis module inefficient in fairly short order. This is illustrated by the fact that Menon et al. find it necessary to use a normally closed valve to prevent flow of water through the reverse osmosis unit except under the circumstances where a "sufficient dynamic pressure drop" is established to provide a vigorous flow through the reverse osmosis unit to prevent scaling. Under conditions of less vigorous flow in the water supply resulting in merely a low pressure drop, the normally closed valving system, plus a pressure sensor which operates the valve, prevents flow through the reverse osmosis unit of Menon et al.
A reason for this problem, which requires the added expense and inconvenience of an automatic, pressure sensitive valving system, is that the reverse osmosis unit of Menon et al. exhibits a generally conventional Void Volume Ratio of about 0.4. The term "Void Volume Ratio" is defined by the Void Volume within the reverse osmosis unit of the unprocessed water flow path upstream of the membrane (i.e., the volume of the "brine" flow channel) divided by the overall internal volume of the reverse osmosis unit, including the membranes, the support screening, and the product water flow channel.
Additionally, the Menon et al. reverse osmosis unit exhibits a Void Volume (defined as above) divided by the membrane surface area of approximately 0.015 cubic inches per square inch, hereafter called the Void Volume Fraction. When the Void Volume Fraction is this low, scaling problems are increased, so that such a reverse osmosis unit is vulnerable to the acquiring of scale if it is exposed to relatively low flow conditions of the type that are eliminated by the normally closed valve and pressure operated control system of Menon et al.
By this invention, a water treatment system is provided which may be installed without major modifications of the main water line, yet which can be conveniently installed under a sink and elsewhere, with relatively small branch lines communicating with the main line. The water flow passing through the treatment system is spontaneous when water is being drawn through the main line, without the need for pressurization pumps or the like. Thus, the water treatment system of this invention can be used with less plumbing modification and with greater flexibility and versatility, for improved ease and convenience of installation. Despite this, the scaling problem is reduced without the need for a valve to automatically limit flow through the reverse osmosis module when the pressure drop is low.